Heat exchangers are used for changing a temperature of various working fluids such as an engine coolant, an engine lubricating oil, an air conditioning refrigerant, and an automatic transmission fluid, for example. The heat exchanger typically includes a plurality of spaced apart fluid conduits or tubes connected between an inlet tank and an outlet tank, and a fin disposed between adjacent conduits. Air is directed across the fins of the heat exchanger by a cooling fan or a motion of a vehicle, for example. As the air flows across the fins, heat in a fluid flowing through the tubes is conducted through the walls of the tubes, into the fins, and into the air.
One of the primary goals in heat exchanger design is to achieve the highest possible thermal efficiency. Thermal efficiency is measured by dividing the amount of heat that is transferred by the heat exchanger under a given set of conditions (amount of air flow, temperature difference between the air and fluid, and the like) by the theoretical maximum possible heat transfer under those conditions. Thus, an increase in the rate of heat transfer under a given set of conditions results in a higher thermal efficiency.
One method typically employed to improve the thermal efficiency of the heat exchanger is to form dimples on the outer surface of the tubes. The dimples form corresponding protrusions on an inner surface of the tubes. The protrusions cause the flow of the fluid within the tubes to be turbulent which is known to increase the heat transfer from the fluid to the tube. However, the dimples typically cause an increase in a pressure drop of the fluid flowing through tubes, in part, because the prior art dimple patterns cause the cross-sectional area of the tubes to vary along the length of the tube. The varying cross-sectional area of the tubes causes a bulk velocity of the fluid to vary along the length of the tubes, which causes the increase in the pressure drop of the fluid. The increase in pressure drop reduces the thermal efficiency of the heat exchanger, and partially offsets the improvement in the thermal efficiency caused by the turbulent flow of the fluid.
The typical dimple pattern interferes also can interfere with the attachment of the radiator fins to the tubes. A brazing process is typically employed to bond one edge of the fins to the outer surface of the tubes. The edge of the fins span the dimples formed in the tube when in alignment therewith. The brazing quality between the fins and the tubes is reduced at the locations where the fins span the dimples, which reduces the heat transfer between the tube and fin, and can reduce the service life of the tube. Additionally, many prior art dimple patterns provide straight line rows of dimples formed on the outer surface of the tubes that, when in alignment with a desired location for a fin, will prevent the brazing of the fin to the tube at the desired location.
It would be desirable to produce a tube for a heat exchanger having a dimple pattern formed thereon, wherein a thermal efficiency of the heat exchanger and a durability of the tube are maximized.